In several cancer diseases, 2-[18F]-Deoxy-D-Glucose (FDG) is applied for monitoring response to therapy. To this end, sequential studies are performed. The uptake of the lesions is determined in a baseline Positron Emission Tomography (PET) study prior to therapy. This uptake is compared to the lesion uptake during therapy. It has been shown that decreasing uptake correlates with better response to the applied therapy. Likewise, the same or even increased tracer-uptake correlates with poor response. Besides FDG, other tracers have been investigated as well, e.g. 18F-fluoromisonidazole (18F-FMISO) and 18F-fluoro-L-thymidine (18F-FLT). Also for these tracers, changes of the tracer-uptake can be correlated to outcome in order to predict response to therapy. Of course, contrast agents for other imaging modalities (SPECT, US, MR) may also be employed in therapy monitoring.
For all diagnostic and monitoring approaches, the clinical protocol applied in the imaging procedures is essential, because the tracer distribution and the lesion uptake are dynamic processes. Immediately after tracer injection, the tracer is found in the blood pool only. The uptake in the lesion increases over a certain period of time until a plateau is reached. After reaching the maximum, the tracer is washed out and the tracer concentration in the lesion decreases again. Depending on the tracer, the disease, and further aspects, the maximum uptake is reached at different times post injection (p.i.). For example, FDG peaks at about two hours p.i. while the maximum FMISO uptake is found after about four hours p.i. To assess response, the optimum measurement would have to be performed at the peak uptake. For logistical reasons, however, much shorter times are applied in clinical practice. For instance, FDG data are usually acquired at sixty or ninety minutes p.i. PET acquisitions are performed comparatively early in the uptake phase. Since there is still a significant slope in the uptake curve at this point in time, variations in the delay between injection and acquisition impact the measured uptake. To complicate matters, the treatment influences the uptake mechanisms so that the uptake curve during therapy may be different from baseline and cause variations in the uptake measurements which may even invalidate the data with regard to the intended comparison of baseline and monitoring scan.
US2007/0066892 discloses a method for providing tomographic pictures of a patient with the aid of a tomographic system by injecting the patient with tracers and subsequently determining temporal concentration profile of the tracers in at least one predetermined body region in at least one scanning plane. The drawback with this reference is that there is no consideration taken to added complexity caused by a variable delay between the tracer injection and the measurement of the lesion uptake.
The inventor of the present invention has appreciated that an improved tracer-uptake measurements is of benefit, and has in consequence devised the present invention.